Electric heaters are commonly used in many applications. However, there are currently no dc heaters that can be heated to up to or more than 100° C. using a voltage of 9 volts or less and in which the dc heater is one inch square or smaller. Such a small heater powered by a low voltage source will open up possibilities for many applications in terms of space and power saving, in particular for portable and even disposable use.
For example, PCR is a commonly used method to make multiple copies of a DNA sequence for various application such as DNA cloning for sequencing, diagnosing disease, identifying individuals from DNA samples, and performing functional analyses of genes. In PCR, replication of the DNA sequence takes place in multiple thermal cycles, with each cycle typically having three main steps: denaturation, annealing and extension. In the denaturation step, a double-stranded DNA template is heated to about 94-98° C. for 20-30 seconds to yield single-stranded DNA. In the annealing step, primers are annealed to the single-stranded DNA by lowering the temperature to about 50-65° C. for 20-40 seconds. In the extension step, using a DNA polymerase (such as Taq), a new double-stranded DNA is synthesized by extending the primer that has been annealed to the single-stranded DNA at an optimum activity temperature of the DNA polymerase (75-80° C. for Taq). Appreciably, replication of the DNA is exponential as the new double-stranded DNA formed in a cycle undergoes denaturation, annealing and extension in the next cycle, such that each cycle effectively doubles the number of DNA sequences obtained. In addition to the three main steps mentioned above, an initialization step may be required if the DNA polymerase used is heat activated, and the final extension step of the last cycle may be held for a longer period of time (e.g. 5-15 minutes) to ensure that there are no remaining single-stranded DNA fragments.
Thus, any device for performing the PCR needs to be able to perform the repeated thermal cycles in order for the steps of denaturation, annealing and extension to take place. This involves heating and cooling the reaction to the required temperatures and holding the required temperatures for the necessary lengths of time. Given that temperatures go up to nearly and/or more than 100° C., existing microfluidic or lab-on-chip PCR devices typically require an external thermal cycler to supply the necessary heat, thereby limiting theft true portability and size during use.
In other applications such as the warming up of blood bags for transfusion use or even simply heating up of food stored in bags commonly known as retort pouches, bulky heating devices requiring significantly higher voltage sources than 9 volts are currently typically used, thereby limiting the portability and convenience for their users.